The field of the invention is optical field sensing and analysis.
Systems and devices that rely upon obtaining intelligence from optical fields are limited by the sensor used to sense the optical field. Image intensity provides limited information, and techniques which analyze only intensity are accordingly limited. More powerful analysis techniques, such as interferometry, seek to expand the information through use of the intensity to determine other aspects of the optical field such as the phase, the polarization, or the spectral information.
The typical sensing device in an imaging system is a photodetector that senses optical intensity. Photodetectors can detect only the intensity of an optical field and cannot detect the phase of the optical field. Therefore, the field parameters that are dependent on the phase of the optical field have to be coded into intensity measurements. For example, interferometers detect phase differences by interfering different optical fields or portions of the same optical field. The resulting interferograms code the phase information into intensity information. Similarly, the temporal spectrum of the optical field consists of different components having different temporal oscillation frequencies. That means that their phases change with different velocities. A regular photodetector cannot output different signals as a function of the frequency of the incident optical field in the spectral regions where it has a flat spectral response. Fourier transform spectrometers (FTS) interfere delayed versions of the optical field and output the resulting intensity measurements. The Fourier transform of the measurements gives the spectral content of the input field.
Interferometers, holograms, spectrometers and other optical instruments code phase related information into intensity information. In some cases, the output intensity is measured for more than one state of the instrument. For example, the path length difference of an interferometer is changed between measurements to provide additional phase information. This is done by moving mirrors, moving gratings, moving wave plates, and other means. These operations may remove degeneracies (like in the estimating the phase angle from its cosine value, which is measured by interferometers) or they may increase the accuracy of the estimate in the presence of noise. The so-called phase-shift algorithms are used for that purpose. In the case of shearing interferometers, the shear direction is changed (from x to y for example) in some cases in order to recover two-dimensional phase maps. A problem arises when the input changes (e.g., a pulsed or variable field) or when the system also changes uncontrollably between the measurements, thus reducing the accuracy of the result. Similarly, complete polarization measurement of the optical field requires four intensity measurements. This multiple shot measurement requires rotating polarizers and waveplates to obtain the polarization information (including the phase lag between TE and TM components) from intensity data. This poses a problem if the input is changing during the time of the measurement. If the input field contains spectral information, Fourier transform spectrometers, such as the Michelson interferometer, may recover the spectral content. Fourier transform spectrometers scan the path/phase delay between the two arms and measure the interference output for different delays. Multiple measurements are required, which poses a problem in the case of changing inputs.
There are other ways to generate more information per output frame. In these cases the output information is not similar to that of multi frame systems although it is used to solve the same problems. For example, crossed gratings have been used to generate simultaneous x and y shears in wave-front sensors. Triple shearing interferometry, which uses array generation techniques to produce three sheared interfering beams from one input beam, has also been proposed for real-time wave front sensing. We proposed a technique that samples the input wave front with an array of small apertures and generates both phase shift and shear diversity information. The phase shift information for both x and y shears was generated by appropriately choosing the diffraction distance and the sampling positions in the output aperture of the sensor. Our approach had limited light throughput making it unusable with reduced input intensities.
The invention is a new method to be used to generate more diversity data per frame of the output of an optical system. The present method can be applied to any situation where phase, polarization, shear, coherence (spatial and/or temporal), and wavelength data is recorded in a single or multiple frames of the output to obtain information from the input optical field. It generates the information in fewer frames, or even one frame of the output. We define a frame of the output to be the measurement of intensity in the output plane of the system taken at one time instance. The data is similar to that in the corresponding multi-shot system. Thus, each particular application of the method possesses the same advantages as the corresponding multi-shot system. In addition, the invention is able to use fewer frames, or even one frame, of the output to reconstruct the information in the input optical field. Also, in contrast to multi-path systems, the information is generated essentially along the same optical path, resulting in more compact systems that are also less sensitive to vibrations and misalignment.
The present invention can be applied to reduce the number of frames in multi-frame diversity based field measurement techniques. It is capable of generating amplitude, phase, polarization, spatial, temporal coherence, and/or wavelength diversity data along an essentially common optical path by using a sparsely sampled input field and generating the diversity data in the empty regions between the samples. The diversity data associated with each sample is generated by imaging the respective samples into multiple appropriately modified and spatially shifted copies, placed in the empty space between the samples and appropriately modified to provide diversity data alone or through interference with the copies coming from the same sample or from different samples in the input. The method can also be used with a continuous input field. The continuous field is first sampled with a sampling device in order to obtain a sparsely sampled version. The diversity data is processed with algorithms parameterized by the characteristics of the fan out.